Stellar Loops

June 17, 1996

Galaxy NGC 55

This spectacular image of a nearby galaxy, NGC 55, captures huge
filaments, loops and "bubbles" of ionized hydrogen gas thousands
of light years across. These structures probably result from
material blown outward by many generations of massive stars in
galaxy. NGC 55 is an irregular galaxy that has hints of spiral
structure; it is similar to the Large Magellanic Cloud, a companion
to the Milky Way that is visible in the southern sky. NGC 55
resides in a relatively nearby gathering of galaxies, the
Sculptor Group, which is located about 5 million light years
from the earth.

Annette Ferguson of Johns Hopkins University presented this new
view of NGC 55 on June 11 at a meeting of the American
Astronomical Society in Madison, Wisc. She and a number of
colleagues created the image using the 1.5-meter optical
telescope at the Cerro Tololo Inter-American
Observatory in Chile. To
capture the many faint details of the galaxy's structure,
Ferguson's team made a composite of observations from September,
1994 and from September, 1995.

Astronomers theorize that the tangled protrusions from NGC 55
resulted from disruptions caused by a class of hot, rapidly
evolving stars within the galaxy. These stars, which have at
least ten times the mass of the sun, shine brilliantly and eject
material in powerful stellar winds. When such massive stars
exhaust their nuclear fuel, they undergo cataclysmic
detonations--supernova explosions--which
blast away the star's outer layers. The combination of stellar
winds and supernova explosions sculpts the interstellar hydrogen
gas into the bubbles and filaments seen in this image.

Similar formations are seen in some other galaxies, most notably
M82, an
object so roiled by stellar evolution that astronomers once
mistakenly considered it an exploding galaxy.

Energetic photons from the hot stars ionize the hydrogen,
removing each atom's single electron. These loose electrons soon
"recombine" with bare hydrogen nuclei (protons), releasing
energy in the process. That energy appears in the form of light
of a specific wavelength--6,563 Angstroms (an Angstrom is one
ten billionth of a meter), which falls toward the red end of
the visible spectrum. Ferguson and her colleagues used a special
filter, limiting their observations to this narrow band of the
spectrum. For comparison, take a look at unfiltered
color and black-and-white images of the galaxy.

Observations such as this one demonstrate that, despite the
successes of the orbiting Hubble Space
Telescope, breathtaking
research can still be done from the ground. If anything, there
has been a resurgence in the building of new terrestrial
instruments that deliver unprecedented clarity at a variety of
wavelengths.

Until recently, the 10-meter Keck
Telescope on Mauna Kea,
Hawaii, was the world's largest optical telescope. Now it has a
twin: on May 8, scientists formally dedicated the Keck II
Telescope, located next to Keck I atop the 13,796-foot dormant
volcano. Keck II will incorporate three specialized
spectrographs--instruments for recording an object's spectrum--
that will complement the capabilities of the five-year-old Keck
I.

Even more exciting, the two telescopes are designed to work
together as an interferometer--a system in which light from two
or more telescope is brought together so that the light waves
combine and interfere with each other. This new technique of
optical interferometry can
yield extremely high-resolution images, essentially allowing the
two telescopes to function as a single, enormous instrument.
Because Keck I and Keck II are some 85 meters (nearly 280 feet)
apart, they will have a resolving power equivalent to that of a
telescope with an 85-meter mirror, or about 0.005 arc seconds at
infrared wavelengths. That is about ten times the precision of
the Hubble Space Telescope.

Also at the June meeting of the American Astronomical Society,
astronomers from the University of Massachusetts, Amherst and
the Instituto Nacional de Astrofosica, Optica y Electronica in
Puebla, Mexico revealed plans to build one of the world's
largest high-precision radiotelescopes. Dubbed the Large
Millimeter Wave Telescope (LMT), the 50-meter
instrument will be constructed over the next five years on a
yet-to-be selected mountain site in Mexico.

Millimeter waves (shorter than the waves observed by conventional
radio telescopes) can penetrate through dust clouds and
interstellar haze, revealing the hidden details of star birth,
planetary formation and other mysterious astronomical processes.
When completed, the LMT's huge dish-shaped antenna will stand
taller than a 16-story building, and be able to point to a
cosmic source with an accuracy of 1/1,000th of a degree (about
four arc- seconds). It will also provide a view of the southern
sky that is relatively unexplored, and which includes the center
of our Milky Way galaxy.